1. Field of the Invention
This invention relates to an apparatus and method for controlling a multiplication factor of light receiving means and controlling a gain of amplifying means for electrical signals corresponding to the received light signals. More specifically, the invention relates to an automatic multiplication factor and gain control apparatus and method which has application in a repeater for an optical digital signal transmission apparatus, etc.
2. Description of the Related Art
In a repeater and a terminal equipment which are used in a optical digital transmission apparatus, a photoelectric converting circuit having a light receiving element, such as an avalanche photodiode, has been used. An automatic gain control apparatus is used for controlling the multiplication factor of the light receiving element and/or the gain of a circuit for amplifying electrical signals corresponding to the light received signals, by detection of peak levels of the electrical signals.
This type of apparatus needs to control two parameters--specifically the multiplication factor and the gain as mentioned above--based on one detecting signal from a detecting circuit which detects the peak levels of the electrical signals.
FIG. 11 shows a block diagram of an automatic gain control apparatus for such a purpose. This control apparatus includes a light receiving element 10 which may be an avalanche photodiode. The receiving element 10 receives optical signals with a signal level Pr, and converts the optical signals into electrical signals. The levels of these electrical signals are dependant on the bias to the receiving element. Output signals from the receiving element 10 are amplified by a pre-amplifier 12 and then by a variable gain amplifier 14. Output signals So from the amplifier 14 are supplied to an output terminal 16 and to a peak detecting circuit 18. The peak detecting circuit 18 detects peak levels of the output signals So over a period of time and generates peak voltage signals corresponding to the peak levels. A difference amplifier 20 compares the peak voltage signals from the peak detecting circuit 18 with a reference voltage signal Vref and produces amplified difference signals Vo corresponding to the difference between the peak voltage signals and the reference voltage signal Vref. An amplifying gain GA of the variable gain amplifier 14 is controlled by the amplified difference signals Vo from the difference amplifier 20. The amplified difference signals Vo are also supplied to a variable voltage circuit 22 which generates variable level signals which are supplied to the receiving element 10 as its bias to control its multiplication factor M. A switching means 24 has one position where the variable level signals are supplied to the receiving element 10, to set its multiplication factor M.
FIG. 12 shows characteristics of the relation between the optical signal level Pr, the multiplication factor M of the receiving element 10 and the amplifying gain GA of the variable gain amplifier 14.
When the optical signal level Pr is at its minimum value (Pr=P min), an optimum value Mopt of the multiplication factor M, at which the signal to noise ratio is maximum, should be set. Therefore, when the optical signal level Pr becomes less than a predetermined value Ps (Pr&lt;Ps), the variable voltage signals are set in the variable voltage circuit 22 so that the factor M can be set to the optimum value Mopt. In FIG. 11, a comparing circuit 26 compares the voltage value of the amplified difference signals Vo from the difference amplifier 20 with a predetermined voltage value V.sub.th. When the voltage value of the amplified difference signal Vo is less than the predetermined value V.sub.th, the switching means 24 is commanded to change from the state shown in FIG. 11 to another state, by output signals from the comparing circuit 26. This another state applies voltage signals, which are set in a temperature compensating circuit 28, to the receiving element 10. A DC voltage converting circuit 30 sets voltage signals for supplying to the temperature compensating circuit 28. When the voltage value of the output signals Vo is greater than the predetermined value V.sub.th, the switching means 24 is set as shown in FIG. 11 to apply the output signal from the variable voltage circuit 22 to the receiving element 10.
This apparatus maintains the multiplication factor M as constant whenever "Pr&lt;Ps". Therefore, the output voltage from the variable voltage circuit 22 may not be increased, even if no optical signal is input to the receiving element 10. This apparatus therefore has the advantage of preventing too high a signal bias, and thereby preventing the possibility of destruction of the receiving element 10. Nonetheless, the applied voltage to the receiving element, corresponding to the optimum value Mopt of the multiplication factor M, is different for each receiving element 10. Also the optimum value Mopt is temperature dependent, and changes corresponding to the changing of the temperature. Moreover, the multiplication factor M may change suddenly around the optimum value Mopt even if the changing of the applied voltage to the receiving element 10 is small. Therefore, in order to keep the multiplication factor M at the optimum value Mopt, it is necessary to accurately compensate the change of the applied voltage to the receiving element 10 due to the change of temperature. This gives rise to the need for temperature compensating circuit 28 and the output voltage from the compensating circuit 28 must be adjusted in accordance with each element so that the multiplication factor M of the element 10 can be kept constant because the characteristic between the applied voltage and the multiplication factor M is different in each element. This makes it complicated to keep the multiplication factor M at the optimum value Mopt in the element 10.